Signaling system



March 9; 1937. J, p, SHANKUN 7 2,073,409

SIGNALING SYSTEM Filed July 18, 1934 QZL 0.5C/LL A 70/? I INVENTOR JOHNP. SHA KLIN ATTORNEY Patented Mar. 9, 1937 PATENT OFFICE SIGNALINGSYSTEM John P. Shanklin, Riverhead, N.

Radio Corporation of America,

of Delaware Y., assignor to a corporation Application July 18, 1934,Serial No. 735,767

2 Claims.

This invention relates to a method and means for producing signalingcurrents, the character of which is representative of signals to betransmitted and a method of and means for converting interruptedsignaling currents into signal indications or markings.

My method of and means for converting signaling currents into signalindications or markings also includes provision for eliminating to alarge extent the effect on the indications or markings of weak signalscaused by fading of the received wave.

The novel features of my invention have been pointed out withparticularity in the claims appended hereto as required by law. Thenature of ing, in

my invention and the manner in which the same is carried out will beunderstood by the following detailed description thereof and therefromwhen read in connection with the attached drawwhich;

Figure 1 serves to illustrate the principle of my invention; while,

Figure 2 illustrates a practical application of my novel method andmeans relating to a receiving circuit. The diagram of this circuit showsonly the essential elements, but there may also be included any otherelements known in the art which will enhance the operation thereof.

In Figure 1 a radio frequency oscillator 01 is coupled to the input of asecond radio frequency oscillator 02. The output of the oscillator 02 iscoupled in turn to the input of a third radio fre quency oscillator 03.The oscillators are of the thermionic type and may include variablefrequency determining means, and frequency controlling means.

With the oscillator 01 inoperative, 02 will oscillate at a frequency f1.If 01 is now put into operation, due to effects which will be explained0 later 02 will operate at a new frequency f2.

Consequently the beat frequency produced between the high frequencyoscillations of Oz and 03 will vary as the oscillator 01 is turned offand on. The oscillator 01 may be turned off in any manner such as bymeans of a key in one of the circuits thereof. By keying the oscillator01 the beat frequency produced by 02 and 03 can be made to vary atsignal frequency.

The principles of my invention are applicable 50 to means for convertingsignaling currents into signal indications or markings as well as tocircuits for producing signaling currents. Such a converting circuit hasbeen shown in Figure 2 in which A represents a signal absorbing system,

55 D1 represents a demodulator of the regenerative type having input andoutput electrodes connected in coupled circuits as shown, the inputcircuit being also coupled to the absorption system A. The regenerativedemodulator D1 has its output variably coupled as shown by a condenser Cto the input circuit of a second regenerative demodulator D2. Thisdemodulator also has its input and output electrodes connected as shownin coupled input and output circuits. Each demodulator has an indicatorconnected as shown in its output circuit. The indicators shown may, ofcourse, be replaced by additional amplifiers and/or recording apparatus.The demodulators D1 and D2 may be normally tuned to the desiredfrequencies by the variable capacities in the input circuits thereof. Inexplaining the operation of this receiver it may be kept in mind thatthe absorption system A may for some purposes be looked upon as theoscillator 01, the demodulator D1 may for some purposes be looked uponas the oscillator 02, while the demodulator D2 may for some purposes belooked upon as the oscillator 03.

By adjusting the coupling between A and D1 and the amount ofregeneration the detector D1 may be caused to produce oscillations at afrequency determined by the elements in the circuit thereof when nosignalis impressed onA, and to fall or be drawn into synchronism withthe signal frequency when impressed on A.

In describing the operation of the receiver of Figure 2 it will beassumed first that the incoming signal is strong enough to pull thedemodulator D1 into synchronism with the signal frequency. Suppose thatwith the signal absent D1 is tuned to oscillate at 1,000 cycles lowerthan the signal frequency, and D2 is tuned to oscillate 1,000 cycleslower than the frequency to which D1 is normally tuned. A 1,000 cyclebeat note will then be heard in the telephones H. On introduction of thesignal in A, D1 will shift to the signal frequency or be entrainedthereby and a 2,000 cycle tone will then be heard in the indicators I-I.These two beat tones (telegraph mark and space) may be separated by anumber of methods such as filters and the operators car. In practice itmight be preferable to use the spacing tone frequency for indicatingpurposes since this tone is heard when no signal is re ceived in A andit is consequently constant in amplitude and in frequency. The secondcondition under which the system must operate is where the signalreceived at A is of reduced strength and insuflicient to entrain or pullthe regenerative demodulator D1 into synchronism therewith. Although thesignal does not entrain the oscillations of D1 and synchronize the same,it will neverthelem cause D1 to shift in frequency. In this case markingand spacing tones will be produced as-in the prior case and may beseparated as before by the indicators H or by a filter circuit coupledto the output of either of the demodulators.

The principle upon which the device operates may be explained asfollows. The change of frequency of the detector D1 when a signal waveis impressed thereon is due to two known effects. The first is thechange of bias on the control grid of detector D1 due to the rectifiedsignal current in the grid circuit thereof added to the rectified gridcurrent already present when the tube is oscillating. This causes thedetector to shift frequency, but not necessarily to synchronize with thesignal frequency. The second effect results from the fact that thesignal excites the detector at the signal frequency and hence tends tosynchronize the detector D1 with itself. If the radio frequency voltagesdue to the signal in the demodulator circuit are greater than the radiofrequency voltages generated by the tube itself, the tube and circuitwill of course synchronize with the signal.

My method and means of signaling and of converting signals into markingand spacing is based on the fact that, under certain conditions, if analternating voltage of one frequency fl is introduced in a circuitalready oscillating at another frequnecy f2 there will be a change ofthe frequency of oscillation of the latter circuit which will nowoscillate at a third frequency f3. f3 may or may not equal f1.

By using the principle of my invention, as explained above in connectionwith Figure 2, it becomes possible to receive a short wave telegraphsignal through a narrow band pass filter, say 2,000 cycles broad. Thisis practically impossible using present day methods. .The use of a bandpass filter gives a very noticeable reduction of static and is usedextensively in long wavetelegraphy. The signal heard when using themethod of reception described herein is of constant amplitude and thefrequency stability of the tone heard depends only on the stability ofthe two local oscillators D1 and D2. The signal heard is not affected,within certain limits, by fading and by frequency shift of the incomingsignal.

The band pass filter may be connected as shown in place of the headphones H in the output of the detector D2.

Having thus described my invention and the operation thereof, what Iclaim is:

1. In a device of the class described a source of modulatedoscillations, a receiving circuit including two cascaded stages ofoscillating demodulators, means for so applying energy from saidoscillation source to the first of said demodulators as to cause itsfrequency to lock in step with the modulated oscillations, and means forso combining the output energies from the two said demodulators as toproduce beat notes of different frequencies depending upon thecharacterization of the modulations from the first said source.

2. In a signaling system having three sources of oscillations, themethod of signaling which includes the steps of deriving oscillationsfrom each of the first two sources, causing the oscillations from thesecond of said sources to be locked in step with oscillations from thefirst of said sources, causing modulations to be applied to theoscillations from the first of said sources at a signaling rate therebyto vary the fundamental frequency of said oscillations, and causingoscillations from the second of said sources when varied in frequency tobe variably heterodyned with oscillations from said third source therebyto produce beat notes the frequency of which changes at the signal rate.

JOHN P. SHANKLIN.

